Over the past few decades, there have been significant advancements in the field of wireless communication. Wireless technology has found applications in a variety of areas such as telephony, control of industrial devices, entertainment and many more. Some common examples of wireless communication systems include mobile phones, cordless phones, pagers, and wireless LANs.
Wireless communication systems typically involve the use of transmitters and receivers for the transmission and the reception of data signals respectively. The data signal is embedded in a carrier wave. The carrier wave is typically a sinusoid whose oscillation frequency is referred to as the carrier frequency. The carrier wave is modulated at the transmitting end according to the characteristics of the data signal. During modulation, a particular characteristic, like amplitude or frequency, of the carrier wave is varied according to the data signal. The carrier wave, which is modulated using the data signals, is termed as a modulated carrier wave. The modulated carrier wave is demodulated at the receiver end to recover the original data signals. Thus, the data signals are exchanged between the transmitter and the receiver.
For effective exchange of data signals in a wireless communications system, it is imperative that both the transmitter and the receiver operate at the same carrier frequency. Consistency of frequency is ensured by using frequency references in devices such as transmitters and receivers. A frequency reference is an oscillator that produces a standard frequency, from which the operating frequencies of the receiver and the transmitter are derived. Typically, a frequency reference is implemented using a piezo-electric crystal. Other types of frequency references, including those constructed of integrated circuit elements such as resistors, inductors and capacitors, tend to be less accurate and less expensive than crystal-based references. In general, the reference elements in the transmitter and the receiver are different and produce slightly different frequencies. Even if the transmitter and the receiver reference elements are of similar design, they may produce different frequencies or vary over time due to manufacturing variation and environmental factors such as temperature, vibration, and aging. This leads to a mismatch between the carrier frequencies at which the transmitter and the receiver operate. This mismatch is termed as frequency offset. The frequency offset between the transmitter and the receiver is a major hindrance in achieving efficient exchange of data signals between the two devices. The effect of this frequency offset needs to be minimized in order to improve the quality of wireless communication. This is known as frequency offset mitigation. The frequency offset mitigation is achieved without affecting the value of the frequency offset.
A number of approaches exist in the art for mitigation of the frequency offset. One such method involves use of a suitable demodulation technique that can mitigate the largest expected frequency offset. The demodulator, using this demodulation technique, carries out demodulation without reducing the frequency offset.
There exist a number of techniques dealing with the frequency offset in wireless communication. U.S. Pat. No. 5,497,400, titled “Signal receiver and method of compensating frequency offset”, assigned to Motorola, Inc., describes a decision feedback demodulator. The decision feedback demodulator is used to extract data from a received signal. The decision feedback demodulator mitigates the frequency and the phase error by rotating the phase of the received signal. The amount of phase rotation is determined with the help of prior phase rotation values.
The techniques discussed above utilize a single demodulator. The use of a single demodulator for all cases of frequency offset results in a cost versus sensitivity tradeoff. A demodulator incorporating a specific demodulation technique has its specific cost and sensitivity value. Whenever the demodulator is used to demodulate a signal, the specific cost is incurred and the specific sensitivity level is achieved. The cost incurred is higher for achieving higher levels of sensitivity. However, the demodulator may be used for a wide range of applications requiring different levels of cost and sensitivity. Hence, the cost incurred may be higher than necessary for applications that can tolerate poorer sensitivity than that provided by the demodulator. Alternatively, applications requiring very good sensitivity will be affected due to poor sensitivity provided by a low cost demodulator. Applications requiring good sensitivity are those that require a high distance separation level such as agricultural monitoring applications. In such cases, higher cost receiver devices incorporating demodulation techniques providing better sensitivity will be a cost effective trade-off because fewer devices will be needed to cover a given geographical area. On the other hand, applications such as a wireless computer mouse or keyboard operate over very short distances and do not require good receiver sensitivity. In this case using a low-cost, low-stability receiver device is appropriate.
From the above discussion, it is evident that there exists a need for a receiver that provides customized frequency offset mitigation. A need also exists for a single receiver that can accommodate a variety of applications, the applications requiring different cost versus sensitivity attributes. The receiver should provide a level of cost and sensitivity best suitable to the application as per its requirements.